1. Field of the Invention
This invention is directed to compounds that provide for sustained systemic concentrations of GABA analogs following administration to animals. This invention is also directed to pharmaceutical compositions including and methods using such compounds.
2. State of the Art
Rapid clearance of drugs from the systemic circulation represents a major impediment to effective clinical use of therapeutic and/or prophylactic compounds. Although multiple factors can influence the systemic concentrations of drugs achieved following administration (including drug solubility, dissolution rate, first-pass metabolism, p-glycoprotein and related efflux mechanisms, hepatic/renal elimination, etc), rapid systemic clearance is a particularly significant reason for suboptimal systemic exposure to many compounds. Rapid systemic clearance may require that large doses of drug be administered to achieve a therapeutic or prophylactic effect. Such larger doses of the drug, however, may result in greater variability in drug exposure, more frequent occurrence of side effects, or decrease in patient compliance. Frequent drug administration may also be required to maintain systemic drug levels above a minimum effective concentration. This problem is particularly significant for drugs that must be maintained in a well-defined concentration window to provide continuous therapeutic or prophylactic benefit while minimizing adverse effects (including for example, antibacterial agents, antiviral agents, anticancer agents, anticonvulsants, anticoagulants, etc.).
Conventional approaches to extend the systemic exposure of drugs with rapid clearance involve the use of formulation or device approaches that provide a slow or sustained release of drug within the intestinal lumen. These approaches are well known in the art and normally require that the drug be well absorbed from the large intestine, where such formulations are most likely to reside while releasing the drug. Drugs that are amenable to conventional sustained release approaches must be orally absorbed from the intestine and typically traverse this epithelial barrier by passive diffusion across the apical and basolateral membranes of the intestinal epithelial cells. The physicochemical features of a molecule that favor its passive uptake from the intestinal lumen into the systemic circulation include low molecular weight (e.g. <500 Da), adequate solubility, and a balance of hydrophobic and hydrophilic character (log P generally 1.5-4.0) (Navia and Chaturvedi, P. R. Drug Discovery Today 1996, 1, 179-189).
Polar or hydrophilic compounds are typically poorly absorbed through an animal's intestine as there is a substantial energetic penalty for passage of such compounds across the lipid bilayers that constitute cellular membranes. Many nutrients that result from the digestion of ingested foodstuffs in animals, such as amino acids, di- and tripeptides, monosaccharides, nucleosides and water-soluble vitamins, are polar compounds whose uptake is essential to the viability of the animal. For these substances there exist specific mechanisms for active transport of the solute molecules across the apical membrane of the intestinal epithelia. This transport is frequently energized by co-transport of ions down a concentration gradient. Solute transporter proteins are generally single sub-unit, multi-transmembrane spanning polypeptides, and upon binding of their substrates are believed to undergo conformational changes, which result in movement of the substrate(s) across the membrane.
Over the past 10-15 years, it has been found that a number of orally administered drugs are recognized as substrates by some of these transporter proteins, and that this active transport may largely account for the oral absorption of these molecules (Tsuji and Tamai, Pharm. Res. 1996, 13, 963-977). While in most instances the transporter substrate properties of these drugs were unanticipated discoveries made through retrospective analysis, it has been appreciated that, in principle, one might achieve good intestinal permeability for a drug by designing in recognition and uptake by a nutrient transport system. Drugs subject to active absorption in the small intestine are often unable to passively diffuse across epithelial cell membranes and are too large to pass through the tight junctions that exist between the intestinal cells. These drugs include many compounds structurally related to amino acids, dipeptides, sugars, nucleosides, etc. (for example, many cephalosporins, ACE inhibitors, AZT, etc).
Gamma (“γ”)-aminobutyric acid (“GABA”) is one of the major inhibitory transmitters in the central nervous system of mammals. GABA is not transported efficiently into the brain from the bloodstream (i.e., GABA does not effectively cross the blood-brain barrier). Consequently, brain cells provide virtually all of the GABA found in the brain (GABA is biosynthesized by decarboxylation of glutamic acid with pyridoxal phosphate).
GABA regulates neuronal excitability through binding to specific membrane proteins (i.e., GABA receptors), which results in opening of an ion channel. The entry of chloride ion through the ion channel leads to hyperpolarization of the recipient cell, which consequently prevents transmission of nerve impulses to other cells. Low levels of GABA have been observed in individuals suffering from epileptic seizures, motion disorders (e.g., multiple sclerosis, action tremors, tardive dyskinesia), panic, anxiety, depression, alcoholism and manic behavior.
The implication of low GABA levels in a number of common disease states and/or common medical disorders has stimulated intensive interest in preparing GABA analogs, which have superior pharmaceutical properties in comparison to GABA (e.g., the ability to cross the blood brain barrier). Accordingly, a number of GABA analogs, with considerable pharmaceutical activity have been synthesized in the art (See, e.g., Satzinger et al., U.S. Pat. No. 4,024,175; Silverman et al., U.S. Pat. No. 5,563,175; Horwell et al., U.S. Pat. No. 6,020,370; Silverman et al., U.S. Pat. No. 6,028,214; Horwell et al., U.S. Pat. No. 6,103,932; Silverman et al., U.S. Pat. No. 6,117,906; Silverman, International Application No. WO 92/09560; Silverman et al., International Application No. WO 93/23383; Horwell et al., International Application No. WO 97/29101, Horwell et al., International Application No. WO 97/33858; Horwell et al., International Application No. WO 97/33859; Bryans et al., International Application No. WO 98/17627; Guglietta et al., International Application No. WO 99/08671; Bryans et al., International Application No. WO 99/21824; Bryans et al., International Application No. WO 99/31057; Belliotti et al., International Application No. WO 99/31074; Bryans et al., International Application No. WO 99/31075; Bryans et al., International Application No. WO 99/61424; Bryans et al., International Application No. WO 00/15611; Bryans, International Application No. WO 00/31020; Bryans et al., International Application No. WO 00/50027; and Bryans et al, International Application No. WO 02/00209).
Pharmaceutically important GABA analogs include, for example, gabapentin, pregabalin, vigabatrin and baclofen. Gabapentin is a lipophilic GABA analog that can pass through the blood-brain barrier, which has been used to clinically treat epilepsy since 1994. Gabapentin also has potentially useful therapeutic effects in chronic pain states (e.g., neuropathic pain, muscular and skeletal pain), psychiatric disorders (e.g., panic, anxiety, depression, alcoholism and manic behavior), movement disorders (e.g., multiple sclerosis, action tremors, tardive dyskinesia), etc. (Magnus, Epilepsia, 1999, 40:S66-S72). Currently, gabapentin is also used in the clinical management of neuropathic pain. Pregabalin, which possesses greater potency in pre-clinical models of pain and epilepsy than gabapentin is presently in Phase III clinical trials.
Rapid systemic clearance is a significant problem with many GABA analogs including gabapentin, which consequently require frequent dosing to maintain a therapeutic or prophylactic concentration in the systemic circulation (Bryans et al., Med. Res. Rev., 1999, 19, 149-177). For example, dosing regimens of 300-600 mg doses of gabapentin administered three times per day are typically used for anticonvulsive therapy. Higher doses (1800-3600 mg/day in divided doses) are typically used for the treatment of neuropathic pain states.
Sustained released formulations are a conventional solution to the problem of rapid systemic clearance, as is well known to those of skill in the art (See, e.g., “Remington's Pharmaceutical Sciences,” Philadelphia College of Pharmacy and Science, 17th Edition, 1985). Osmotic delivery systems are also recognized methods for sustained drug delivery (See, e.g., Verma et al., Drug Dev. Ind. Pharm., 2000, 26:695-708). Many GABA analogs, including gabapentin and pregabalin, are not absorbed via the large intestine. Rather, these compounds are typically absorbed in the small intestine by the large neutral amino acid transporter (“LNAA”) (Jezyk et al., Pharm. Res., 1999, 16, 519-526). The rapid passage of conventional dosage forms through the proximal absorptive region of the gastrointestinal tract has prevented the successful application of sustained release oral dosage technologies to GABA analogs. Thus, there is a significant need for effective sustained release versions of GABA analogs to minimize increased dosing frequency due to rapid systemic clearance of these compounds.
Another deficiency with some GABA analogs, including gabapentin, is their lack of dose-proportional oral bioavailability (see Radulovic et al, Drug Metab. Dispos. 1995, 23, 441-448; Gidal et al, Epilepsy Res. 2000, 40, 123-127; Gabapentin Supplementary Basis for Approval, Warner-Lambert, Inc.). Absorption of gabapentin in mammals is subject to saturation, since the large neutral amino acid transport system has limited substrate capacity and is localized to the upper part of the small intestine, creating an absorption window that restricts the ability of the drug to be taken up into the bloodstream. Thus in man, gabapentin oral bioavailability decreases from about 60% at a dose of 300 mg to about 35% at a dose of 1600 mg. This leads not only to inefficient use of the administered drug, but also to unpredictable and highly variable drug levels in patients, particularly at the higher doses associated with efficacy in the treatment of epilepsy and neuropathic pain (Gidal et al, Epilepsy Res. 1998, 31, 91-99). There is, therefore, a need for derivatives of gabapentin and other GABA analogs, which following oral administration to a patient in need of therapy provide therapeutically efficacious levels of the GABA analog in the plasma of a patient, where the concentration of the GABA analog in plasma of the patient over time provides a curve of concentration of the GABA analog in the plasma over time, the curve having an area under the curve (AUC) which is substantially more proportional to the dose of GABA analog administered, as compared to the proportionality achieved following oral administration of the GABA analog itself. There is similarly a need for derivatives of gabapentin and other GABA analogs, which following oral administration to a patient in need of therapy provide therapeutically efficacious levels of the GABA analog in the plasma of a patient, where the concentration of the GABA analog in plasma of the patient over time provides a curve of concentration of the GABA analog in the plasma over time, the curve having a maximum plasma concentration (Cmax) which is substantially more proportional to the dose of GABA analog administered, as compared to the proportionality achieved following oral administration of the GABA analog itself.
One pathway that might provide for the sustained delivery of drugs with rapid systemic clearance is the proton-coupled peptide transport system (Leibach and Ganaphthy, Ann. Rev. Nutr. 1996, 16, 99-119). These transporters mediate the cellular uptake of small intact peptides consisting of two or three amino acids and are found primarily in the intestine and kidney. In the intestine, where small peptides are not well-absorbed by passive diffusion, the transporters act as a vehicle for their effective absorption. Transporters in the kidney actively reabsorb di- and tri-peptides from the glomerular filtrate, thereby increasing their half-life in the circulation.
Two proton-coupled peptide transporters that have been cloned and characterized are PEPT1 and PEPT2. PEPT1 is a low-affinity, high-capacity transporter found primarily in the intestine. The human PEPT1 consists of 708 amino acids and possesses 12 putative transmembrane domains. PEPT2, in contrast, is a high-affinity, low-capacity transporter found mostly in the kidney. It consists of 729 amino acids and is 50% identical to human intestinal PEPT1.
Studies of PEPT1 and PEPT2 have shown that the transporters account for the absorption and reabsorption of certain therapeutically active compounds. The compounds include both biologically active peptides (e.g., renin inhibitors) and zwitterionic antibiotics. Based on these observations, researchers have suggested that peptide transporters, in conjunction with cytosolic peptidases, could be exploited for systemic delivery of certain drugs in the form of peptide prodrugs (see Tsuji and Tamai, Pharm. Res. 1996, 13, 963-977). Dipeptide analogues of α-methyldopa, L-α-methyldopa-Phe and L-α-methyldopa-Pro, for example, are absorbed more efficiently in the intestine than α-methyldopa alone. Once across the intestinal membrane, the dipeptides are hydrolyzed by cytosolic peptidases to release α-methyldopa.
Gallop et al have provided evidence from transporter mRNA expression profiling studies that PEPT expression in rat and human extends broadly over the length of the intestine, including the colon (U.S. Patent Application Ser. No. 60/351,808 filed 24 Jan. 2002). They have suggested that sustained exposure to a substrate for a PEPT transporter could be achieved by formulating such a compound in an extended-release dosage form, which would gradually release the compound during transit of the formulation through the large intestine.
Peptide prodrug derivatives of gabapentin and other GABA analog drugs are contemplated by Bryans et al (see International Application No. WO 01/90052; U.K. Application GB 2,362,646; European Application EP 1,178,034). These workers have disclosed gabapentin derivatives wherein the amino group is blocked with particular α-aminoacyl or dipeptide moieties. More specifically, the α-amino acids comprising these peptide prodrug derivatives are the 20 naturally encoded α-amino acids, plus phenylglycine.
Prodrug derivatives of gabapentin and other GABA analog drugs are also disclosed by Gallop et al (see the co-pending International Applications WO 02/28881, WO 02/28883, WO 02/28411 and WO 02/32376). The compounds disclosed therein are bile acid conjugates of GABA analogs that are designed to be actively transported across the intestinal mucosa via interaction with the ileal bile acid transporter. These conjugates are further designed to undergo enterohepatic recirculation and to slowly release the parent GABA analog into the systemic circulation. Additional prodrug derivatives of gabapentin and other GABA analog drugs are disclosed by Gallop et al (see the co-pending International Application WO 02/42414). The compounds disclosed therein are α-aminoacyl and β-aminoacyl conjugates of GABA analogs that are designed to be actively absorbed across the intestinal mucosa via interaction with peptide transporters expressed in the intestine.